- Glasgow-haskell-users

Fun with type functions
by Simon Peyton-Jones 27 Nov '08

27 Nov '08

Friends GHC has embodied data type families since 6.8, and now type synonym families (aka type functions) in 6.10. However, apart from our initial papers there isn't much published material about how to *use* type families. But that hasn't stopped you: quite a few people are using them already, and of course there is a rich seam of work on using functional dependencies to express type-level computation. Ken Shan and Oleg Kiselyov and I are collaborating to write a paper for an upcoming workshop, under the general rubric of "Fun with type functions" (in homage to Thomas Hallgren's paper "Fun with functional dependencies" and Ralf Hinze's paper "Fun with phantom types"). So this message is to ask you: can you tell us about the most persuasive, fun application you've encountered, for type families or functional dependencies? Simple is good. It doesn't have to be elaborate: just something that does something useful you could not have done otherwise. Pointers to email threads are fine. Don't assume we already know about them (even if we participated in the thread :-) Part of what we're interested in is that *you* found the example compelling. Many thanks Simon, Ken, Oleg PS: I'm broadcasting this message to GHC-users and Haskell-cafe, but to avoid deluging ghc-users, please reply just to us and Haskell cafe. (Interested ghc-users can follow the threads there from the archives if they want.)

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Linking to Haskell code from an external program
by Colin Paul Adams 26 Nov '08

26 Nov '08

I am trying to call a Haskell function from an Eiffel program, using C as an intermediary. For starters, I compiled and ran a variation of the program shown in http://haskell.org/haskellwiki/Calling_Haskell_from_C, to make sure I had the C-code right. I then attempted to move it into Eiffel. I can compile the C code OK, but I'm running into problems with linking. I solved most of the problems by adding the -v flag to the call to ghc which I used to link the original (haskell + c only) program, and cut-and-paste the linker options from their into the Eiffel configuration file. This isn't really satisfactory - I would like some automatic way to determine what the flags should be. The remaining problem has me stumped. I am getting the following messages: big_file_C4_c.c:(.text+0x9f4f): undefined reference to `__stginit_Fib' /home/colin/fib/Fib_stub.o: In function `stginit_export_Fib_zdffibonaccizuhszuaQO': Fib_stub.c:(.text+0x5): undefined reference to `Fib_zdffibonaccizuhszuaQO_closure' /home/colin/fib/Fib_stub.o: In function `fibonacci_hs': Fib_stub.c:(.text+0x32): undefined reference to `Fib_zdffibonaccizuhszuaQO_closure' The Haskell code looks like this: >{-# LANGUAGE ForeignFunctionInterface #-} > > module Fib where > > import Foreign.C.Types > import CString > > fibonacci :: Int -> String > fibonacci n = show (fibs !! n) > where fibs = 0 : 1 : zipWith (+) fibs (tail fibs) > > fibonacci_hs :: CInt -> IO CString > fibonacci_hs = newCString . fibonacci . fromIntegral > > foreign export ccall fibonacci_hs :: CInt -> IO CString And the C code (in an Eiffel wrapper) looks like this: haskell_fib (n: INTEGER): POINTER is -- String representation of `n'th Fibonacci number as a string external "C inline use <stdio.h>,%"HsFFI.h%"" alias "[ #ifdef __GLASGOW_HASKELL__ #include "Fib_stub.h" extern void __stginit_Fib ( void ); #endif int argc; char *argv[0]; argc = 0; hs_init(&argc, &argv); #ifdef __GLASGOW_HASKELL__ hs_add_root(__stginit_Fib); #endif return (fibonacci_hs($n)); hs_exit(); ]" end I can't find any mention of the word "fibonacci" in the output from the ghc run, and I don't know where this _closure stuff is coming from. Can you help? -- Colin Adams Preston Lancashire

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Grammar for (Glasgow) Haskell
by Colin Paul Adams 26 Nov '08

26 Nov '08

Is this available as a happy file (or something similar)? I tried searching the 6.10.1 source package for *.y, but the three files I found don't seem to be what I'm looking for. What I want to try to do is to generate C wrappers to call GHC-compiled code by parsing foreign export declarations. But I figure just searching the code for a regular expression on the lines of foreign[ ]+export is going to be over-simplistic. But I'd rather not type in all the Haskell grammar myself (I'm lazy - you wouldn't expect any other sort of person on this list, I assume :-) ). -- Colin Adams Preston Lancashire

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Exporting an #include from one package to another?
by Conal Elliott 26 Nov '08

26 Nov '08

Is there a way to package up an #include like to share across (be #include'd into) various other haskell packages? I have some standard templates for type class instances that I can't define as instances without creating lots of overlapping instances. I can write an include file easily enough, but I don't know how to get other packages to find and #include that include file. - Conal

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Future plans: unicode and line editing
by Ian Lynagh 25 Nov '08

25 Nov '08

Hi all, We've been weighing up the options to solve the recent problems that editline has given us, and we think that this is the best way forward: For 6.12: * http://hackage.haskell.org/trac/ghc/ticket/2811 Implement unicode support for text I/O (we've had this on the TODO list for some time now; it was slated for 6.10, but didn't make it) * http://hackage.haskell.org/trac/ghc/ticket/2812 Switch to using haskeline instead of editline Haskeline should be able to use the above unicode support, rather than the utf8-string package * http://hackage.haskell.org/trac/ghc/ticket/2813 Create (and ship with GHC) a utf8 bytestring-a-like I'm not sure if this is actually needed by either of the above, but it is certainly related, and something we want to do anyway We won't be trying to get any of the above into future 6.10 releases. However, for 6.10 you can use Judah's ghci-haskeline: http://code.haskell.org/~judah/ghci-haskeline/ which uses the GHC API and the haskeline package to implement the GHCi frontend. If you have any problems using ghci-haskeline, then it is likely that GHCi in 6.12 will have the same problems, so please let us know! Comments and criticisms welcomed! Thanks Ian

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Re: Can't compile GHC 6.8.2
by John Lato 25 Nov '08

25 Nov '08

> On Monday 24 November 2008 23:15, Barney Stratford wrote: >> There's good news and bad news. The good news is that the compilation of >> my shiny almost-new GHC is complete. The bad news is, it won't link. >> It's grumbling about >> >> ld: >> /System/Fink/src/fink.build/ghc-6.8.2-1/ghc-6.8.2/rts/libHSrts.a(PrimOps.o) >> has external relocation entries in non-writable section (__TEXT,__text) >> for symbols: >> ___gmpn_cmp ----------- etc. ---------- IMO, many apple dev. tools are insidiously broken (Apple's cpp is my current nemesis). Could you try using a different linker? That might be a sufficient workaround. You could call gcc with the correct arguments to complete the linking phase, rather than ld (or vice-versa). The only other suggestion I've seen is to re-order the exported functions before compiling the library. I don't know if this would help in your case, but it has worked with other libraries. That's the best I can suggest, anyway. John

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Backspace/delete/history in ghci
by Colin Paul Adams 24 Nov '08

24 Nov '08

I can't use the backspace key, or arrow keys in ghci (6.10.1) as i would normally expect to on any program on Linux. Is this connected with the readline/editline/haskelline debate I have observed recently? -- Colin Adams Preston Lancashire

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Re: ANNOUNCE: GHC version 6.10.1 - EditLine / terminal incompatibility?
by Philip K.F. Hölzenspies 24 Nov '08

24 Nov '08

Dear Ian, all, After the announcement, I downloaded and installed the final release of 6.10.1. At some prior stage, I used the RC1 release, which had the same bug, but I had generally assumed it to be an RC-phenomenon. Which bug? Well... It seems editline has some incompatibility with my terminal. Maybe, I should configure something differently, but any and all of the control characters I type and work for all readline based (and all custom line-readers I have come across), but not for the new ghci. Most annoying is the fact that backspace doesn't work (instead, the control sequence ^? is printed and pressing enter results in "lexical error at character '\DEL'"). However, arrow keys (left ^[[D, up ^[[A, right ^[[C and down ^[[B) and common Ctrl+_ combinations (^d, ^e, ^a) also give similar errors. It is striking that the lexer does recognize them as the correct characters (printed error messages doe say \DEL, \EOT, \ENQ, etc). I'm using xterm (X.Org 6.8.99.903(229)) on an OpenSuSE 10.3 system and the default termcap settings. Any clues? Is this bug known? Regards, Philip

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sum[1..100000] --> stack overflow in GHC, but not in Hugs?
by Vishal Belsare 23 Nov '08

23 Nov '08

Hi, Please bear with a very basic question. I am trying to 'learn me a Haskell for great good' using Hutton's book and some online tutorials. I started off with Hugs and recently used GHC (to use the 'let a = .." syntax interactively, which Hugs doesn't allow perhaps). There's a simple issue I am having with GHC. sum[1..100000] works fine on Hugs, however, the same in GHCi generates a stack overflow exception. I am trying to figure out what's going on. Prelude> sum[1..100000] 5000050000 Prelude> sum[1..1000000] *** Exception: stack overflow Prelude> ________________________ Hugs> sum[1..10000] 50005000 Hugs> sum[1..100000] 5000050000 Hugs> Thanks in anticipation. Best regards, Vishal Belsare ps: please excuse if the question is too uncool for the list. I come from an economics background; the markets are bad, I am depressed and to add to my pain, GHC refuses to let me know sum[1..100000] -- Firstly, I propose for myself the rule that the obviousness of a proposition does not free me from the obligation to continue searching for a proof of it, at least until I clearly realize that absolutely no proof could ever be required, and why. ~ Bernhard Bolzano

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Unicode's greek lambda
by Kazu Yamamoto 22 Nov '08

22 Nov '08

Hello, When the -XUnicodeSyntax option is specified, GHC accepts some Unicode characters including left/right arrows. Unfortunately, the letter "greek lambda" cannot be used. Are there any technical reasons to not accept it? Regards, --Kazu

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